The present invention relates to a switching power supply, and more specifically, relates to a technique for reducing the size of the switching power supply.
Conventionally, a semiconductor integrated circuit for the switching power supply has been developed, in which a power terminal and a feedback terminal are made common to reduce the number of terminals, thereby realizing a small size of an external shape of a package and reduction of external parts. The switching power supply constructed by using the semiconductor integrated circuit is disclosed in, for example, Japanese Patent Application Laid-Open No. H5-137327. FIG. 1 is a block diagram of a configuration of a conventional switching power supply.
The switching power supply includes a transformer 10, a control circuit 200 connected to a primary winding 20 of the transformer 10, a rectifying and smoothing circuit 270 connected to a drive winding (tertiary winding) 210 of the transformer 10, a photo transistor 110a connected to an output side of the rectifying and smoothing circuit 270, a rectifying and smoothing circuit 170 connected to a secondary winding 30 of the transformer 10, an output voltage detection circuit 180 connected to an output side of the rectifying and smoothing circuit 170 and including a photo diode 110b, a load 190, and a capacitor 100 for stabilizing a supply voltage, which is connected to a power terminal (to be commonly used as the feedback terminal) of the control circuit 200.
The photo transistor 110a is optically coupled with the photo diode 110b to form a photocoupler 110.
The control circuit 200 is formed of a semiconductor integrated circuit, and connected to the primary winding 20 of the transformer 10, to which a DC voltage is inputted. The control circuit 200 includes a switching element 220, an activation circuit 230, a PWM control circuit 240, a detection resistor 250, a detection resistor 260, and a start circuit 280.
The activation circuit 230 supplies a constant current bias temporarily to the PWM control circuit 240, immediately after power on. The supply of the bias current by the activation circuit 230 is intercepted after sufficient energy is supplied to the power terminal from the rectifying and smoothing circuit 270 connected to the drive winding 210.
When the voltage of the power terminal rises due to the supply of bias from the activation circuit 230 to reach a predetermined value, the start circuit 280 activates the PWM control circuit 240. As a result, a pulse of voltage is outputted from the drive winding 210, and the pulse of voltage is rectified and stabilized by the rectifying and smoothing circuit 270. The output of the rectifying and smoothing circuit 270 is supplied to the capacitor 100 for stabilizing the power supply via the photo transistor 110a. Accordingly, the supply voltage V100 is supplied to the PWM control circuit 240.
A signal fed back from the output voltage detection circuit 180 on the secondary side via the photocoupler 110 is superimposed on the supply voltage V100, so as to control the voltage such that the voltage rises under a light load condition, and drops under a heavy load condition. A voltage obtained through resistive potential division of the supply voltage V100 by the detection resistor 250 and the detection resistor 260 is supplied to the PWM control circuit 240, and hence, the PWM control circuit 240 controls an on duty width of the switching element 220. As a result, a constant voltage is outputted from the rectifying and smoothing circuit 170 on the secondary side.
The semiconductor integrated circuit constituting the control circuit 200 can reduce the number of control terminals to 3 by integrating the power terminal and the feedback terminal, thereby enabling installation thereof in a small package for a power transistor. Accordingly, the semiconductor integrated circuit is particularly used for a small switching power supply such as a charger for a mobile phone.